pkg subdir for srckit-std

Current version of `packages/XEmacs.rules' assumes old `packages' subdirs structure in its `srckit-std' target. More precisely, it assumes that there are `$(CATEGORY)' subdirs, where `$(CATEGORY)' is `standard', `mule', `wp' and so on. If fact currently there are `{xemacs,mule}-packages' subdirs. One has to do some tricks to generate package source distribution. If it is known that all packages with `CATEGORY = mule' are in `mule-packages' and others are in `xemacs-packages' then the makefile rule may be fixed by adding a trivial `case' statement.